1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, and a computer-readable recording medium.
2. Description of the Related Art
In the field of commercial printing, the method for performing, for example, small lot printing, multi-variety printing, and variable data printing is shifting from the conventional method of using an offset printing machine to a POD (Print On Demand) method using an electrophotographic type image forming apparatus. In order to fulfill various needs, there is a demand for the electrophotographic type image forming apparatus to provide performance (e.g., precision in front/back orientation, consistency of image formation) equivalent to that of the offset printing machine.
The factors causing front/back registration slur in the image forming apparatus can be categorized into i) registration error in a vertical direction/horizontal direction, ii) skew error between a recording medium and a printed image, and iii) expansion/shrinkage of an image (increase/decrease of image length) when transferring the image. Further, in a case where an image forming apparatus includes a fixing apparatus, the heat applied from the fixing apparatus to a recording medium may cause the expansion/shrinkage of the recording medium. As a result, the expansion/shrinkage of the recording medium may cause image magnification error and lead to front/back disorientation.
In view of the above, there are methods for reducing magnification error of the front and back sides of a recording medium by calculating image magnification of the front and back sides of a recording medium based on expansion/shrinkage amounts of the recording medium before/after performing a fixing process on the recording medium and correcting image data according to the expansion/shrinkage amounts of the recording medium (see, for example, Japanese Patent No. 4227367, Japanese Laid-Open Patent Publication No. 2004-129069, and Japanese Patent No. 4111026).
However, with the conventional methods for correcting image magnification, after an image is printed on a front side of a recording medium, it is necessary to calculate expansion/shrinkage amounts of a recording medium before/after performing a fixing process and perform magnification correction for an image to be printed on the back side of the recording medium before the image to be printed on the back side of the recording medium is formed on a photoconductor drum or the like.
With a typical image forming apparatus, a certain amount of time is required until an image is transferred to a recording medium. That is, an image is transferred to a recording medium by forming an electrostatic latent image on a photoconductor drum, developing the electrostatic latent image into a toner image by using a development apparatus, and rotating the toner image together with the photoconductor drum. In order to utilize the results of measuring the expansion/shrinkage amount of the recording medium to an image forming process, a measuring unit for measuring the expansion/shrinkage amount of the recording medium is needed to be sufficiently separated from an area of a recording medium conveyance path at which a toner image is transferred to the recording medium.
However, after the recording medium has been shrunk by the heat of the fixing apparatus of the image forming apparatus, the recording medium changes shape in a recovering direction. Therefore, the size of the recording medium changes from the time of passing the measuring unit and the time when a toner image is transferred to the recording medium. Accordingly, it is difficult to perform high precision magnification correction on an image to be printed on a recording medium.
In view of the above, it is possible to stop conveying a recording medium after measuring the length of the recording medium (expansion/shrinkage amount of the recording medium) immediately before transferring a toner image on the recording medium, so that the toner image can be formed in accordance with the measured expansion/shrinkage amount. However, this method may degrade productivity of the image forming apparatus.